Human skin processes occurring as a result of exposure to UV radiation, emanating either from the sun or from an artificial light source, have been investigated thoroughly over the past year. Erthema, the well known sunburn response of human skin, occurs as a result of exposure to UV radiation of wave lengths in the region of 250-400 nm (UV-C, UV-B, and UV-A), and disappears with time. Factors found to influence the erythemal process include the individual skin type, the initial skin pigmentation, and the sun protection factor (SPF) of the tanning lotion being used. Recent research reviewed by McKinlay and Diffey ("A reference action spectrum of ultra-violet induced ethyema in human skin", presented at the UV Hazards Conference, Amsterdam, 1987, pp. 83-87) has focused on determining the skin's erythema action spectrum, the relative sensitivity of the skin to erythema as a function of the wave length of the radiation to which the skin has been exposed.
The MED (minimal erythema dose) has been defined by medical research as the minimal dose of radiation capable of producing a detectable erythema reaction. However, repeated exposure to radiation below the MED can have an accumulative effect in producing sunburn. The accumulative effectiveness of exposure below the MED has been found to linger for 12-24 hours.
Various devices have been proposed for monitoring the erythemal and/or tanning effect of UV radiation (U.S. Pat. Nos. 4,485,306; 4,428,050; 4,010,372; 3,710,115; 4,348,664; and 2,114,163, West German 3,126,127). Some of these devices are capable of simulating the skin's relative responsiveness to UV radiation as a function of the wavelength of that radiation, as indicated by the erythema action spectrum. None of the device described in the above patents has been successful in accurately simulating the sensitivity of the skin to UV radiation as a function of wavelength in accordance with the erythema action spectrum without being unacceptable dependent on the angle of incidence of the UV radiation.
Simply stated, the prior art UV exposure sensors have not been able to overcome the inherent limitation in optical filters operative with a desired transmissivity/wavelength relationship in the region of interest, namely that such filters are extremely sensitive to the angle of incidence of the incoming UV radiation.
Accordingly, the prior art devices are not suitable for ordinary use by the public in outdoor environments wherein accurate positioning of the sensor is not readily achieved.